Integrated circuit technology utilizes photoresist compositions which generally comprise a polymeric resin or binder compound, a radiation sensitive compound, and a suitable solvent to enable forming a film of the composition over a particular substrate for photolithographically delineating patterns on such substrate. In a typical processing scheme, the photoresist compositions are spun on or applied with different methods known in the art to the substrate. Then the photoresist compositions are subjected to a pre-exposure bake to drive off a proportion of the solvent to impart dimensional stability to the film. The coated substrate is exposed with actinic radiation, usually in the UV, e-beam or x-ray spectra, using an appropriate exposure tool for such exposure. After exposure, the coated substrate undergoes a development process where, due to selective dissolution of certain areas, a pattern is formed or developed. In certain areas of the photoresist film, the resist material is completely removed while in other areas the remaining photoresist forms a pattern having a desired or intended configuration. Such patterns are used to mask or protect the substrate for subsequent wet or dry etching processes, the deposit of conductor or insulator patterns, or for incorporation of the patterned photoresist into the device or package as, for example, an insulating or passivating layer.
The remaining resist material is further exposed to dry or wet etching. This process is essential to define or transfer the pattern and to enable further processing such as the deposition of dielectric or metallic film to be patterned by, for example, a lift-off or polishing step later on in the process sequence. Before this etching process the photoresist materials may be treated with a blanket exposure with deep UV radiation, at very high temperatures ranging from 140.degree. to 300.degree. C. for 30 seconds to 90 seconds. This treatment improves the resist selectivity and provides better etching and dimensional controls.
It is necessary in a comprehensive lithographic process that the photoresist material, following pattern delineation, be evenly and completely removed in certain areas and retained in others, in order to permit further operations. Any portion of photoresist remaining in a region of desired removal for such processing may cause deleterious results. Photoresist residues can lead to defects which are detrimental to yield.
In the past, resist materials have been removed by one or more of the following: halogenated hydrocarbons, such as methylene chloride or tetrachloroethylene; amines and their derivatives, such as dimethylformamide, N-methyl-2-pyrrolidone; glycol ethers such as ethylene glycol monomethyl ether, ethanol and the acetates thereof; ketones such as methyl ethyl ketone and acetone; and materials such as isopropyl alcohol, sulfuric acid, ammonium persulfate, dioxane, and mixtures or caustic and phenol derivatives as well as various other materials. These various agents suffer one or more disadvantages such as potential toxicity to workers exposed to such materials, environmental and pollution problems after use, volatility, corrosion of equipment, and the safety hazards inherent in operating at temperatures above the flash point of the stripper. Additionally, as the ground rules on semiconductor devices and packaging call for an even greater degree of integration, the presence of even traces of resist materials has become less tolerable. Therefore, processing with strippers heretofore known has involved metal attack and/or attack of the silicon substrate itself. The presence of water and/or high temperatures (&gt;100.degree. C.) exacerbate such attacks.
Processing conditions for resists such as high temperature post exposure bake or pattern generation, the use of techniques such as ion implantation (e.g., boron ion implantation and the like), and deep ultraviolet radiation hardening lead to highly crosslinked resists which are extremely resistant to dissolution in most organic solvent strippers conventionally employed in the art. Particularly tenacious compositions containing phenolic or chlorinated hydrocarbon solvents have been employed at elevated temperatures to strip such resists. Such extreme measures are clearly undesireable since they involve considerable hazard to technicians carrying out the stripping as well as presenting potential pollution and environmental problems in the disposal of the resulting waste products. These chlorinated solvent-containing compositions are particularly sensitive to the presence of water which forces the equilibrium to shift to a higher chloride ion concentration in solution which causes metal attack. Aqueous solutions of strong bases have the deleterious side effect of exhibiting aggressive metal attack as well as having an etchant effect on silicon substrate and the like.
The art is replete with attempts to provide improved stripper compositions which have been designed to meet the deficiencies that their inventors perceived. Among the solutions are included the following:
U.S. Pat. No. 4,744,834 is directed to compositions useful for removing photoresists from substrates comprising 10-90% of a 2-pyrrolidone which may be N-alkyl or N-hydroxyalkyl substituted where the alkyl portion has 1-3 carbons, 10-30% of diethylene glycol monoalkyl ether where the alkyl group has 1-4 carbons, 1-10% of a polyglycol having a molecular weight from about 200-600, and 0.5-4% of a quaternary ammonium hydroxide of the form NR.sub.3 R.sub.4 R.sub.5 R.sub.6 where patterns R.sub.3 and R.sub.4 are the same or different alkyl groups of 1-4 carbon atoms, R.sub.5 is an alkyl group of 1-18 carbon atoms, and R.sub.6 is an alkyl group of 1-18 carbon atoms, phenyl, benzyl, alkylphenyl, or alkylbenzyl where the alkyl portion has 1-18 carbons. The compositions of this reference were shown to be useful in the removal of photoresist which was hard baked at 125.degree. C. (a range of 120.degree.- 300.degree. C. is given) by use of the stripping solution at 90.degree. C.
U.S. Pat. Nos. 4,395,479, 4,401,748 and 4,428,871 are directed to 2-pyrrolidone based stripping compositions which include a tetrahydrothiophene-1,1-dioxide, a tetrahydrofuran, or a diethylene glycol monoalkyl ether respectively to provide improved stripping results. These compositions may also preferably include propylene glycol and a diethylene glycol monoalkyl ether in the case of the first two compositions.
U.S. Pat. Nos. 4,765,844 is directed to resist stripper compositions comprising 10-100% of a difunctional water-soluble amino derivative which is either a diamino or an amino hydroxy compound and 0-90% of a propylene glycol derivative.
U.S. Pat. No. 4,776,892 discloses processes for removing photoresist compositions from a substrate including the use of an aqueous solution containing an organic quarternary ammonium base, preferably tetramethylammonium hydroxide (TMAH). The solutions may also contain a strong inorganic base. The stripping is generally carried out at an elevated temperature of 30.degree.-70.degree. C.
U.S. Pat. No. 3,673,099 is directed to resist stripping compositions and processes which provide a composition comprising a mixture of N-methyl-2-pyrrolidone (NMP), and a strong base such as an alkyl or substituted alkyl ammonium hydroxide. Miscible organic solvents such as ethylene glycol monomethyl ether may also be included. The method is practiced at from 25.degree. C. to the boiling point of the stripping composition.
U.S. Pat. No. 3,706,691 is directed to solvents to dissolve potting (encapsulating) compositions having bond susceptible of hydrolytic cleavage (e.g., polyamides, polyesters, and polyester base type polyurethanes) which comprise benzyltrimethylammonium hydroxide and a constituent selected from the group of (a)tetrahydrofuran and acetone, (b) methylene chloride, (c) methylene chloride and acetone, and (d) N-methyl-2-pyrrolidone.
U.S. Pat. No. 4,239,661 discloses aqueous stripping solutions containing 0.01-20% by weight of trialkyl(hydroxy-alkyl) ammonium hydroxide (THAH) where the alkyl groups have 1-4 carbons and the alkylene portion of the hydroxyalkyl has 2-4 carbons. The THAH solution is very basic and dissolves alkali metals and aluminum and etches silicon oxide.
U.S. Pat. No. 4,617,251 is directed to a stripping composition that is free of phenol compounds and of halogenated hydrocarbon compounds because of the toxicity and disposal problems that accompany such materials. The stripper compositions comprise an amine and an organic polar solvent with a boiling point greater than 140.degree. C.
PCT publication WO 88/05813 (based on PCT application PCT/US87/02291) discloses photoresist stripper compositions comprising a mixture of (a) pyrrolidone, N-substituted pyrrolidone, butyrolactone or caprolactone and (b) about 2-10% by weight of a tetraalkylammonium hydroxide or a trialkylaralkylammonium hydroxide. The ratio of the component from (a) to the component from (b) is from 50:1 to 4:1. Optional ingredients include surfactants and diluents which may include polar organic solvents such as glycols, e.g., ethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, tetrapropylene glycol, etc.